The R group of the amino acid Asp contains a carboxyl group with a pKa of 3.7. Bu the pka of the alpha-carboxyl group in Asp is 1.9. Which of the following. explanations could reasonably account for this difference? 3.7 is the isoelectric pH, which is estimated by averaging the pka of the alpha- carboxyl group with the much higher pKa of the alpha-amino group O The R group of Asp also contains an NH2 group, which is less acidic than a carboxylic acid The conjugate base of the alpha-carboxyl could be stabilized by an attractive ionic interaction with the nearby alpha-amino group, which is protonated at low pH Water has better access to the R group than to the alpha-carboxyl group, so it more effectively solvates the charged form of the R group and stabilizes it

For two samples of cocaine, A was made up in an aqueous solution of pH 2.8, while B was made up in an aqueous solution of pH 9.8. The pka of cocaine is 8.6. Which of the two solutions is ready for cocaine extraction with hexane? Why?

75mL of a 0.2M solution of a tripeptide Ser-Cys-His is adjusted to pH=8.6. The solution is then titrated with 50mL of 0.5M HCl. Calculate the new pH value of the tripeptide solution. (Relevant pKa values are: 2.2, 6.0, 8.3, and 9.6).

50 mL of a 0.1M solution of tripeptide His-Arg-Ala is adjusted to its isoelectric point (pI). The solution is then titrated with 150 mL of 0.05M HCl. Then the solution is titrated with 65 mL of 0.1M NaOH. Calculate the final pH. (Relevant pKa values are 2.1, 6.0, 9.5 and 12.5.)

Calculating the pl for an amino acid without an ionizable side chain is straightforward. Simply the average of the pKa values for the amine and the carboxylic acid. For example, for glycine, the calculation is as follows: pl = pKa-cooH + pKa-NH, 2 2.34 + 9.60 2 = 5.97 For amino acids with ionizable side chains, the pl is calculated by averaging the pKa values of either the two acidic groups or the two basic groups. For example, for glutamic acid, which has two acidic carboxylic acid groups, the calculation is as follows: Calculate the PI for the amino acid lysine (Lys). Calculating the pl for peptides and proteins is slightly more complicated but follows the same general concept.

If the pKa of a weak acid is 5.65, then at what pH will there be a 50:50 mixture of the protonated and the deprotonated forms of the molecule? 5.35

The triprotic form of the amino acid cysteine is shown below, along with the pK, value for each ionizable site. The isoelectric point (pl) for cysteine occurs at pH= 5.1. B 10.8 CH₂ SH C13 A LA -OH (1) What is the charge on the molecule at the isoelectric point? (2) In the predominant form of the molecule at the isoelectric point, is each of the sites (labeled A, B, and C) protonated or unprotonated? site A site B: site C

A student added a total of 75 mL sodium hydroxide to titrate an amino acid solution with 0.5M NaOH. They also calculated that they added a total of 14 mL of sodium hydroxide to arrive at pH 2.6 and a total of 32 mL of sodium hydroxide to arrvive at pH 4.8.What was the initial concentration of the amino acid solution in milimolar (mM)? The calculations must be presented such that those can be recapitulated by the marker. Provide the answer with no decimal places. Show all details of your working out. State the answer in a full sentence.

Consider a "tripeptide" with the following features (at pH = 10.8) • The N-terminal amino acid is leucine • The middle amino acid is aspartate • The C-terminal amino acid is lysine i. What is the 1-letter symbol for each of these amino acids il. Calculate the net charge of the predominant ionic state of this tripeptide at pH 10.8. Show work.

A mixture of three proteins with pl = 7.5, 6.5, and 5.5 is separated using a DEAE-Sepharose column by eluting (washing) with buffer and NaCl. The column is eluted with aqueous buffer with a pH of 8.0, and an increasing [NaCl]. Which of the following proteins will elute from the column last? A The protein with pl = 5.5 B The protein with pl = 6.5 ☑ The protein with pl = 7.5 D These three proteins will not be separated, and will elute together. Your answer